Retrotransposons are a ubiquitous, diverse and ancient component of eukaryotic genomes. A basic understanding of the dynamics of retrotransposon regulation and propagation is crucial to our understanding of the impacts these elements have on the evolution of the genome. Li is the most prominent retrotransposon family in mammals, and one of the most extensively studied groups of elements. Recently, a phylogenetically related group of retrotransposons, the Swimmer elements (L1sw), were isolated and characterized in a group of teleost fishes. Several striking contrasts are noted in the prevalence and sequence diversity of mammalian and fish Li elements in their respective genomes. These distinctions may involve subtle differences in some form of competitive and/or regulatory interactions between the host and the elements, or among the elements themselves. Phylogenetic comparisons indicate that L1sw elements may exhibit features related to regulation and propagation that are ancestral to the mammalian LI family. The L1sw family provides an essential comparative system for studying the dynamic relationships between elements and genomes in vertebrates. However, before we can elucidate the nature of the interactions that shape the lineage structure of these groups of retrotransposons, further study of the evolution of L1sw elements in fish genomes is required. The objectives of this proposal lay the foundation for further work in this system. These are 1) to assess the L1sw lineage diversity in representative teleost species at multiple taxonomic levels, 2) to survey the sequence diversity and distributions of individual element copies at the population and conspecific levels in order to assess the tempo of lineage evolution, and 3) to address several basic questions regarding the molecular evolution of L1sw lineages, It is anticipated that this work will further resolve the nature of the differences in the evolution of Li elements in fish and mammalian genomes and lead to new insights regarding the interactions between the host genomes and the retrotransposons that reside within.